The present invention relates to an electrically conductive rubber roller utilized as a charging means for copying machines, laser beam printers and the like.
In electrophotographic recording widely used in copying machines, laser printers and the like, recording is effected through a series of such processes, such as charging, exposure, development, image transfer, discharging and cleaning, as shown in FIG. 3. In addition, FIG. 4 shows an example of an electrophotographic recording device. The numeral 1 denotes an original table glass; 2 denotes a copy lamp; 3 denotes a selfoc lens; 4 denotes a photoreceptor; 5 denotes a charging roller; 6 denotes an image transfer roller; 7 denotes a developing roller; 8 denotes a paper feed roller; 9 denotes a pad; 10 and 11 denote resist rollers; 12 denotes a developing cartridge; 13 denotes a toner seal; 14 denotes a cleaning blade; and 15 denotes a waste toner box.
As for the charging process, wide use has heretofore been made of a corona charging method in which a wire called a corona charger is subjected to a high voltage. This corona charging method has the following problems: As corona charging proceeds, the photoreceptor and other parts are deteriorated by active molecules such as ozone. These active molecules are also said to be harmful to the human body. Further, this method requires a high voltage source of thousands of volts, causing potential danger due to high voltage. Further, the maintenance aspect is fraught with the problems of wire contamination and wire breakage.
Thus, recently, instead of the corona charging method, which is fraught with problems as described above, practical use has been made of a contact charging method using an electrically conductive rubber roller. In this contact charging method using an electrically conductive rubber roller, the electrically conductive rubber roller which is externally impressed with voltage is contacted with the photoreceptor surface, whereby an electric charge is directly injected into the photoreceptor surface to charge the latter to a predetermined potential.
The above-mentioned contact charging method is also applied to the image transfer process. That is, a toner image formed on the photoreceptor surface is transferred onto the transfer paper sheet surface to which a voltage of a few kilovolts is applied by the electrically conductive rubber roller which is a transfer means contacted with the backside of the transfer paper sheet surface, so as to form an electrostatic charge on the transfer paper sheet surface, whereby the toner image formed on the photoreceptor surface is transferred to the transfer paper sheet surface.
References
(1) Japanese Patent Application Disclosure No. 302777/1970. PA1 (2) "Charging and transfer process", page 47, from the Electrophotography Society's 30th Training Course Text "From Fundamentals of Elements of Electrophotographic Technology to Recent Trends". PA1 (3) "Contact Type Charging Technique Not Using Corona Charger", page 573, from Electrophotography Society's journal, vol. 27, No. 4 (1988).
In said contact charging method, use has heretofore been made of a sponge roller in the form of EPDM (ethylene propylene rubber) used as an electrically conductive rubber roller whose resistance has been adjusted to 10.sup.7 -10.sup.10 .OMEGA. by applying carbon black thereto, which is a common means for imparting electric conductivity.
However, in the case of said electrically conductive rubber roller, the resistance greatly changes to 5.times.10.sup.7 -10.times.10.sup.7 .OMEGA. in high temperature high humidity (32.degree. C. and 85% relative humidity) environments and to 10.sup.9 .OMEGA. in low temperature low humidity (15.degree. C. and 10% relative humidity) environments (See Japanese Patent Application Disclosure No. 302777/1970. It is believed that the reason is that, in high humidity environments, electrically conductive elastomer usually tends to absorb water to lower its electric resistance. With said electrically conductive rubber roller, therefore, changes in electric resistance due to the environment are so large that the charge potential becomes unstable under the influence of environmental changes, thereby failing to provide good image quality.
Actually, when printing was effected by a laser beam printer (Laser Shot Printer A-404, produced by Canon Inc.) by using said electrically conductive EPDM rubber roller, the following result was obtained which was believed to be due to changes in the electric resistance of the electrically conductive rubber roller.
(1) Faults in image quality appeared in the solid portion in high temperature high humidity and low temperature low humidity environments. (2) When dry paper is printed, faults in image quality appeared in the solid portion.